Nondescript Mrs. Green
What you remembered about Mrs. Green was just how forgettable she was. Average height. Nondescript face. Boring drab clothes. No one looked at her twice and that was a mistake.
Do not read a book by its cover. Mrs. Green looked like a commonplace housewife. Actually, her forgettable characteristics oddly gave her the edge. So one year in her olive green indescribable coat, she could be a government spy. The next year, she could be a covert diamond thief. And the next, a leader for operations in exotic countries.
Presently, the mysterious Mrs. Green would just play at being boring.